Abstract

Scientists, managers, and policy-makers need functional and effective metrics to improve our understanding and management of biological invasions. Such metrics would help to assess progress towards management goals, increase compatibility across administrative borders, and facilitate comparisons between invasions. Here we outline key characteristics of tree invasions (status, abundance, spatial extent, and impact), discuss how each of these characteristics changes with time, and examine potential metrics to describe and monitor them. We recommend quantifying tree invasions using six metrics: (a) current status in the region; (b) potential status; (c) the number of foci requiring management; (d) area of occupancy (AOO) (i.e. compressed canopy area or net infestation); (e) extent of occurrence (EOO) (i.e. range size or gross infestation); and (f) observations of current and potential impact. We discuss how each metric can be parameterised (e.g. we include a practical method for classifying the current stage of invasion for trees following Blackburn’s unified framework for biological invasions); their potential management value (e.g. EOO provides an indication of the area over which management is needed); and how they can be used in concert (e.g. combining AOO and EOO can provide insights into invasion dynamics; and we use potential status and threat together to develop a simple risk analysis tool). Based on these metrics, we propose a standardized template for reporting tree invasions that we hope will facilitate cross-species and inter-regional comparisons. While we feel this represents a valuable step towards standardized reporting, there is an urgent need to develop more consistent metrics for impact and threat, and for many specific purposes additional metrics are still needed (e.g. detectability is required to assess the feasibility of eradication).

Notes

Acknowledgments

This paper resulted from the workshop “Tree invasions—patterns & processes, challenges & opportunities” held in Bariloche, Argentina in 2012. We thank all participants at the meeting for valuable discussion. Daniel Simberloff and three reviewers provided valuable comments that improved the manuscript. JRUW acknowledges funding from the South African Working for Water Programme of the Department of Environmental Affairs. IAD was supported by Core funding for Crown Research Institutes from the New Zealand Ministry of Business, Innovation and Employment’s Science and Innovation Group. AP is funded by Ministry of Economy, ICM P05-002 and Conicyt, PFB-23. DMR acknowledges support from the National Research Foundation (Grant 85417), the DST-NRF Centre of Excellence (partly though the collaborative project with the Working for Water programme on “Research for Integrated Management of Invasive Alien Species”) and the Oppenheimer Memorial Trust. CH was supported by the CPRR 81825 of the NRF. BDM was supported by NSF- WildFIRE PIRE, OISE 09667472. BLW was supported by the CSIRO Climate Adaptation Flagship. RDZ was supported by CNPq-Brazil and The University of Tennessee.

Threat: If potential area is multiplied by impact get to ZAR 100 billion year−1.

Survey method(s) used: Systematic walked transects over ~700 ha to generate point distributions. At a national scale this distinctive species has been included in general field-guides for invasive plants for many years, and dedicated leaflets asking for sightings have been distributed nationally since 2009. Any records should also have been picked up by the substantial on-going research, surveillance, and management into Australian acacias in South Africa.

Status: Invasive; E under Blackburn; All four subspecies of lodgepole pine (contorta, bolanderi, latifolia and murrayana) have been planted (Miller and Ecroyd, 1987) and all regenerate naturally. (Ledgard 2001) (in cultivation?): Not known to be cultivated recently. Introduced in 1880 and established widely for erosion control during 1960s and 70s on a few thousand hectares and self-sustaining since then (Miller and Ecroyd 1987, Ledgard 2001). Suggested as possible covering ~100,000 ha by late 1990s (Ledgard 2001).

Potential: all already invasive. 10–15 % of New Zealand land area (i.e. >2.5 M ha) suitable although could be greater.

Abundance: Various density stands. Seeds freely to high elevation and cones relatively young.

Population growth rate: Published information on estimated extent of cover (Miller and Ecroyd 1987, Ledgard 2001) suggests extent may be increasing at between 5 and 8 % per annum despite control efforts.

Extent: Numerous populations (many large and >1,000 hectares) totalling >100,000 ha extent at all densities. Many populations are found in remote locations as a legacy of where their establishment attempted to protect erosion-prone land from mass-movement. Due to their remoteness and potential cost there is little incentive address control or removal.

Threat: Highest threat is in conservation grasslands and alpine zone where removal will have high non-target impacts.

Survey method(s) used: No national objective survey or monitoring. One province (Canterbury Regional Council) has systematic estimates of extent of cover and density in 11 representative catchments ~70,000 ha to generate point and polygon distributions. Department of Conservation records the presence of weed species in a 10 × 10 km grid.

Hui C, Richardson DM, Visser V and Wilson JRU (2014) Macroecology meets invasion ecology: performance of Australian acacias and eucalypts around the world foretold by features of their native ranges. Biol Invasions 16. doi:10.1007/s10530-013-0599-4

Münzbergová Z, Hadincová V, Wild J and Kindlmannová J (2013) Variability in the contribution of different life stages to population growth as a key factor in the invasion success of Pinus strobus. PLoS ONE 8Google Scholar